Answer: The strong nuclear force.
Explanation: The strong force holds the protons and neutrons together in the nucleus. Despite being the strongest force, it has a very small range. (Think about how small an atom is) The strong force is thought to occur due to the exchange of pions between the protons and the neutrons. Since pions have a very short lifespan, the protons and neutrons must be very close to each other in order to exchange them. This phenomenon is what keeps the protons and neutrons so tightly binded together in the nucleus.